This invention relates to a small loop antenna and especially to a tunable small loop antenna which includes a variable capacitive element connected in a series with the loop conductor.
Recently, the demand for small antennas which can be installed in television receivers, radio receivers or can be used as an external portable antenna system, has been growing in the field of consumer electronics. Such demand is also growing in the field of traveling wireless communications, such as taxi radio communications and citizen band transceivers because the size of the transmitters and receivers, incorporated in these systems, are becoming smaller due to the remarkable developments made with integrated circuits.
Generally, the size of the antenna is related to the wavelength of the radiowaves employed. The longer the wavelength, the larger the antenna size. This invention relates to small antennas, the maximum length of which is not more than one tenth of the wavelength used. Accordingly, hereinafter, the term "small antenna" refers to antennas having a maximum length of not more than one tenth of the wavelength employed. The maximum size of the loop antenna according to the invention is defined here as the maximum length between two opposite outer edges of the loop conductor. For example, in the case of circular loop antenna (e.g., FIG. 6) the maximum size is the outer diameter of the loop conductor; in the case of a square loop antenna (e.g., FIG. 10) it is the diagonal length measured from its outer edges.
A variety of small loop antennas includes the tuned small loop antenna. Tuned loop antennas have a fixed capacitive element connected in series with a one-turn loop conductor. The value of the capacitive element and the inductance of the loop is selected so that the circuit is tuned to the desired frequency of the radiowaves employed. One example of such an antenna is shown in U.S. Pat. No. 3,641,576. This antenna is formed on a disc substrate by printed circuit techniques. It has a diameter of approximately 5 inches and is small enough for use in portable radio equipment. This antenna, however, is designed to have a low loaded "Q" value of not more than 10 so as to cover a wide range of FM frequencies. Low "Q" antennas have low gain and, consequently, the sensitivity of such an antenna is low. It is well known to persons skilled in the art that antennas with high sensitivity, and therefore high gain, can be provided by designing the antenna with a high loaded Q value. Such antennas, however, have a narrow bandwidth and are impractical for transmitting or receiving radio or television broadcasting signals which require the wide band coverage.
To overcome the disadvantages of conventional small loop antennas mentioned above, it is possible to utilize a variable capacitance as the capacitive element connected in series with the loop conductor; the variable capacitance can then be adjusted to tune in the desired frequency. Changing the capacitance, however, produces an undesirable change in the input impedance of the antenna.
Therefore, it is difficult to establish the requisite impedance matching between the antenna and the constant standard impedance of the feeder line. One obvious method of correcting this problem is to mechanically adjust, each time the capacitance is varied, the separation of the antenna input/output taps which are coupled to the feeder line. This mechanical adjustment is not desirable, however, for two reasons. First, the tap design (e.g., slidable contact) to accomplish the precise separation would be costly and complicated. Second, the additional resistance necessarily added by a slidable contact design would cause a decrease in the gain and sensitivity of the antenna.